Slice the green onion diagonally into 5mm pieces.

Cut the chicken thigh into bite-sized pieces.

In a pot, combine mirin, salt, soy sauce, and water. Heat over medium heat until boiling, then add the chicken. Skim off any foam, cover, and simmer on low heat for 3 minutes until the chicken is cooked.

Add the green onion to the pot, cover, and simmer for another 2 minutes.

In a separate pot, boil water and cook the Inaniwa udon noodles according to package instructions. Rinse under cold water and drain.